Intrusion detection device utilizing low frequency sound waves and phase detection techniques

ABSTRACT

A signal in the low audio frequency range and preferably of about 400 to 600 hertz is radiated into one or more protected areas by one or more speakers, and each speaker also acts as a transducer of reflected audio energy. A substantially amplitude independent phase sensitive detector designed to operate at the audio frequency is coupled to the source of audio signals and to each speaker to produce an output signal when a reflected signal exhibits phase shift due to even relatively slight movements of an intruder in any direction in the protected area. A low band pass circuit couples a narrow low frequency band of signals from the detector to gating and integrating circuits to provide an alarm indication in response to phase changes characteristic of the intruder movement to be detected.

UTILIZING LOW FREQUENCY SOUND WAVES AND PHASE DETECTION TECHNIQUESHarold Eisenberg, Chicago, Ill.

Assignee: Webster Electric Company, Inc.,

Racine, Wis.

Filed: Dec. 13, 1971 Appl. No.: 207,466

Related US. Application Data Continuation-impart of Ser. No. 147,985,May 28,

Inventor:

US. Cl. 340/258 A, 340/3 D Int. Cl. G08b 13/16 Field of Search 340/1 R,3 D, 3 E,

References Cited UNITED STATES PATENTS 6/1971 Peterson et a1. 6/ 1971Rollwitz et al 5/1957 Wood et a]. 2/1951 340/3 D 340/3 D 340/3 D .Iaynes340/258 A Aug. 21, 1973 3,394,342 7/1968 Walker 340/30 2,066,156 12/1936Muffly ..340/31) Primary Examiner-John W. Caldwell AssistantExaminer-Marshall M. Curtis Attorney- Richard D. Mason, Philip M.Kolehmainen et a1.

[5 7] ABSTRACT A signal in the low audio frequency range and preferablyof about 400 to 600 hertz is radiated into one or more protected areasby one or more speakers, and each speaker also acts as a transducer ofreflected audio energy. A substantially amplitude independent phasesensitive detector designed to operate at the audio frequency is coupledto the source of audio signals and to each speaker to produce an outputsignal when a reflected signal exhibits phase shift due to evenrelatively slight movements of an intruder in any direction in theprotected area. A low band pass circuit couples a narrow low frequencyband of signals from the detector to gating and integrating circuits toprovide an alarm indication in response to phase changes characteristicof the intruder movement to be detected.

13 Claims, 1 Drawing Figure INTRUSION DETECTION DEVICE UTILIZING LOWFREQUENCY SOUND WAVES AND PHASE DETECTION TECHNIQUES The presentapplication is a continuation-in-part of copending application Ser. No.147,985, filed May 28, 1971.

The present invention relates to an improved alarm for providing anindication of the presence of an intruder in a protected area.

Intruder detection systems utilizing ultrasonic, microwave and radiofrequency energy are well known. One type of system introduces radiatedenergy into a protected region and detects changes in incident radiationdue to the presence of an intruder in the region. A widely used systemis the type which mixes the reference or radiated signal with thereceived signal in a diode mixer, or envelope detector, to produce analarm condition when the frequency of the received signal differs fromthe frequency of the radiated signal due to the presence of an intruder.Such a system can react to changes in the amplitude of the receivedsignal as well as to frequency shifts occurring due to the Dopplereffect as the intruder moves toward or away from the signal radiated.One example of this known type of system is disclosed in U.S. Pat. No.Re 25,100.

Known systems of this type are subject to several drawbacks. One seriousproblem is the occurrence of false alarms due to a wide variety ofextraneous causes. Systems using amplitude responsive detectors can beplaced in a false alarm condition by incidental energy of a frequencysimilar to that radiated by the system, which incidental energy can becaused by any number of external sources. Moreover, the use ofultrasonic and higher frequencies leads to problems of false alarmscaused by objects and disturbances other than the presence of anintruder to be detected. For example, small moving objects such asrodents and fastmoving objects such as fans and other equipment canreflect ultrasonic and higher frequency energy and cause false alarms.In addition, small movements such as vibrations of windows and walls dueto traffic noise or other causes, and even temperature changes or airmovement can trigger this type of alarm. Another difficulty is that highfrequency energy is not readily confined in a region such as a room orenclosure wherein protection is desired.

Since an alarm condition may require the travel of security personnelfrom a remote location in order to check the protected area, it can beseen that false alarms can represent a serious expense. The problem offalse alarms with known equipment is so severe that in manyinstallations it is necessary substantially to reduce the sensitivity ofthe equipment. However, when sensitivity of the system is reduced to thedegree necessary to eliminate a large proportion of false alarms, thesystem may no longer be capable of reliably detecting the presence of anintruder.

Many of these difficulties are overcome in accordance with the presentinvention by the use of a relatively low frequency audio signal such asa signal between 100 and 1,000 hertz and preferably between 400 and 600hertz. This low frequency audio signal is readily reflected byconventional walls, drapes and the like and thus is readily adapted tovarious rooms and enclosures. In addition, the signal can be radiated inmany cases with pre-existing paging or intercom speakers. In addition,small objects such as rodents are not effective reflectors of signalshaving long wave lengths. The audio signal is easily detected andrecognized by an intruder, and thus in itself discourages intrusion.

However, detectors used in known intrusion alarms exhibit thecharacteristic that they provide a useful output signal only when motionin the order of magnitude of several wave lengths of the radiated signaltakes place. Consequently, conventional detectors impose the requirementthat at least ultrasonic frequencies, if not higher frequencies, be usedbecause lower frequencies have longer wave lengths and would permit anintruder to move large distances without detection.

Among the important objects of the present invention are to provide animproved intrusion alarm system overcoming the many disadvantages ofknown ultrasound, microwave and radio frequency devices; to provide asystem which is highly reliable in detecting the presence of an intruderand which avoids many causes of false alarms; to provide a system whichis easily installed and maintained; and to provide a system which iseasily adapted to many types of environments and which is readilycombined with existing paging or intercom equipment.

Another important object of the present invention is to provide animproved detector circuit which is substantially amplitude independentand which is constructed in novel manner to detect changes in phase ofsignals in the low audio range.

Another object is to provide a novel signal processing circuit foroperating a polarized gate in response to input signals of eitherpolarity.

In brief, in accordance with the present invention there is provided anintrusion alarm for detecting movement of an intruder in one or moreprotected areas including a speaker located within each protected area,together with an oscillator for generating low audio frequency signalspreferably in the range of from 400 to 600 hertz. The audio signals areradiated into the protected areas by the speakers, and each speaker alsoreceives reflected audio signals.

In accordance with an important feature of the invention, there isprovided a phase sensitive detector coupled to each speaker and alsocoupled to the oscillator. The detector operates substantiallyindependent of the amplitude of reflected signals to provide an outputsignal when the phase of a reflected signal varies due to movement of anintruder within an area.

The output signal from the phase sensitive detector is coupled through alow band pass device having a band width of only a few hertz, therebyachieving a high signal-to-noise ratio and limiting operation of thesystem to phase changes representative of those caused by an intruderdesired to be detected. A gate circuit is operated by signals receivedfrom the low band pass amplifier to apply energy to an integrator havinga predetermined time delay characteristic such that an alarm circuit isoperated when the system indicates the presence of an intruder in thearea. A signal processing circuit coupled between the low band passdevice and the gate serves to operate the gate in novel manner inresponse to either positive or negative signals.

The invention together with the above and other objects and advantagesthereof may best be understood with reference to the embodimentillustrated in the accompanying drawing.

The single FIGURE of the drawing is a schematic and diagrammaticillustration of an intrusion alarm constructed in accordance with theprinciples of the present invention.

Having reference now to the drawing, there is illustrated an intrusionalarm designated as a whole by the reference numeral and embodying thefeatures of the present invention. In general, the intrusion alarm 20functions to create an alarm condition in response to detection of anintruder within a protected area. Although the present invention isdescribed in connection with the securing of a room or other enclosureagainst unauthorized intrusion by a human intruder, it should beunderstood that principles of the present invention may be applied tomany other uses and environments wherein detection of a predeterminedtype of motion within a given area is desired.

In general, the intrusion alarm 20 includes a speaker 22 adapted to belocated in the room or enclosure or other area to be protected. Anoscillator generally designated as 24 generates a low frequency audiosignal, and a speaker drive amplifier generally designated as 26operates the speaker 22 at the low audio frequency. The illustratedintrusion alarm 20 operates at a frequency of 460 hertz, although it isbelieved that signals within the general frequency range of from 100 to1,000 hertz and preferably within the range of 400 to 600 hertz would beacceptable in carrying out the present invention.

One advantage of the intrusion alarm 20 is that the speaker 22 maycomprise part of a pre-existing intercom system or paging system.Preferably the speaker 22 is a high efficiency horn type speaker, butaltematively a cone speaker may be used. In the latter case, it may beadvisable to use a speaker baffle resonant at the frequency of operationof the oscillator 24. If desired, additional speakers 22 may beconnected in parallel with the illustrated speaker and disposed in otherrooms or enclosures to be protected. Up to at least six parallelspeakers may be used without disturbing the operation of the device 20.

Many important advantages flow from the use of a low freuqency audiosignal within the preferred frequency range of the present invention. Inthe first place, such a signal has a long wave length. Since theacoustic energy reflected from objects moving within the protected areadepends on the size of the object relative to the wave length, theintrusion alarm 20 is not substantially affected by rodents or othersmall moving objects which are a cause of frequent false alarms insystem known to the prior art. The long wave length, on the other hand,is ideally suited for detecting motion of a human intruder. Anotheradvantage of a signal within the preferred frequency range is that theradiated energy is easily confined within a room or other enclosure bywalls, windows, drapes and the like of conventional structure. Moreover,the use of a relatively low frequency in the audio range provides adeterrent to intrusion in the protected area and thus preventsundesirable confrontation situations between an intruder and securitypersonnel.

Returning now to a general description of the intrusion alarm 20, inorder to obtain a sensitive, fast and reliable output signal when motionof an intruder takes place in a protected area, there is provided anovel phase sensitive detector generally designated as 28. The detector28 is constructed to operate at the low audio frequency of the intrusionalarm 20 and provides an output signal due to phase shift of thereflected signal received by speaker 22 while being substantiallyindependent of amplitude variations in the reflected signal. Since anymovement of an intruder within a protected area will produce phaseshifts, even if the movement is along a line equidistant from thespeaker 22, the detector 28 provides a reliable indication of movementwithin the room. Since the detector 28 is not sensitive to amplitudevariation of the reflected signal, the intrusion alarm 20 is not subjectto false alarms due to incident noises from extraneous sources to theextent of many prior art devices.

The phase sensitive detector 28 is coupled to the speaker 22 so that itreceives a composite signal made up of the vector sum of the radiatedsignal and the reflected signal received by the speaker 22. In additionthe detector 28 is also connected to a reference amplifier generallydesignated as 30 so that it receives a signal from the oscillator 24unaffected by the reflected signal received by the speaker 22. The phasesensitive detector 28 is highly sensitive and is capable of providing anoutput signal due to phase shift without the necessity of the intrudermoving through a distance corresponding to multiple wave lengths of thegenerated signal.

In accordance with the present invention, the output signal from thephase sensitive detector 28 is coupled through an impedance matchingamplifier generally designated as 32 to a low band pass device generallydesignated as 34. Device 34 comprises an amplifier having a band passcharacteristic such that it couples signals in about a five hertz rangewith sharp cutoff characteristics for signals above the pass band. As aresult, the device 34 filters out signals provided by the detector 28which are not characteristic of phase changes produced by motion of anintruder desired to be detected. Thus, false alarms clue to such factorsas traffic or thunderstorm induced vibrations and the like are largelyavoided.

Connected to receive signals from the low band pass device 34 is a gatecircuit generally designated by the reference numeral 36 serving toapply energy to an integrator generally designated as 38 during suchtime as the signals are coupled through thelow band pass device 34. Theintegrator 38 is selected to have a time I constant such that itoperates a relay control circuit generally designated as 40 in responseto a predetermined accumulation of energy which could result from asubstantial number of small motions or alternatively from a single largemotion. An alarm circuit 42 is operated by the relay control circuit 40when an alarm condition is initiated.

Proceeding now to a more detailed description of the intrusion alarm 20,the oscillator 24 may comprise any type of signal generator capable ofproducing signals in the preferred frequency range. As illustrated, theoscillator 24 includes a transistor 44 with its emitter electrodecoupled to ground through a resistor 46 and with its collector electrodecoupled to a tank circuit including resistors 48, 50 and 52 andcapacitors 54, 56, and 58. The tank circuit is coupled to the baseelectrode of transistor 44 by a capacitor 60, and operating potentialfor the oscillator 24 is coupled to the collector electrode through abias resistor 62, and to the base electrode through a voltage dividercircuit including resistors 64 and 66.

One advantage of the intrusion alarm 20 of the present invention is thatthe frequency of operation of the oscillator 24 does not require specialstabilization. Minor relatively slow changes such as might be caused bytemperature variations and the like do not result in false alarms.However, it is desirable to isolate operation of the oscillator 24 fromthe remainder of the intrusion alarm 20. In the illustrated arrangement,this function is carried out by the drive amplifier 26 and the referenceamplifier 30.

Oscillating output signals of the oscillator 24 appearing at thecollector electrode of the transistor 44 are coupled to the driveamplifier 26 through a resistor 68 and a coupling capacitor 70, and tothe reference amplifier 30 through a resistor 72 and a couplingcapacitor 74. Amplifier 26 includes a transistor 76 with its baseelectrode biased to an operating potential by a voltage divider circuitincluding resistors 78 and 80. The emitter electrode of transistor 76 iscoupled to ground through a resistor 82 and a parallel-connectedcapacitor 84. The output circuit of the transistor 76 includes thespeaker 22 coupled between the collector electrode and a source ofpositive potential.

The reference amplifier 30 may be similar in construction and includes atransistor 86 having a base electrode biased to an operating potentialby a voltage divider including resistors 88 and 90 and an emitterelectrode coupled to ground by a resistor 92. The collector electrode oftransistor 86 is coupled to a positive source of operating potential bymeans of a capacitor 94.

When the audio signals are radiated into the protected area such as aroom or enclosure by the speaker 22, the signals are reflectedthroughout the protected area in a complex pattern of paths, and a partof the radiated energy is reflected back to the speaker 22. Once asteady state condition is reached, a resultant reflected signal ofrelatively constant amplitude and phase is received at the speaker 22.The speaker 22 serves as a transducer for the reflected signal, and as aresult there is produced at the collector electrode of transistor 76 ofthe drive amplifier 26 a composite signal. This composite signalincludes a large component corresponding to the signal produced byoscillator 24 and radiated by speaker 22. The composite signal alsoincludes a smaller component corresponding to the reflected audio signalreceived by the speaker 22. The vector sum of these two signals is thecomposite signal which appears at the collector electrode of transistor76.

One feature of the invention resides in the fact that the intrusionalarm 20 is not effectively responsive to amplitude variations in thereflected signal. To bring about this result, the speaker driveamplifier is driven substantially to saturation in operation and thusfunctions relative to the detector 28 as a limiter stage. Consequently,although signals appearing at the collector electrode of the speakerdrive amplifier transistor 76 vary freely in phase when movement occursin the protected area, nevertheless changes in amplitude of thereflected signal have only a negligible effect on the composite signal.

In order to provide an output signal indicative of movement in theprotected area, the phase sensitive detector 28 is provided. Detector 28includes a transformer 100, and reference signals unaffected by signalsreflected to the speaker 22 are applied to the detector by a transformerprimary winding 102 coupled between a source of positive potential andthe collector electrode of the reference amplifier transistor 86. Thecomposite signal including the reflected signal component is applied toa transformer secondary winding 104 through a centertap 106 and acoupling capacitor 108. In order to operate the detector 28 at optimumsensitivity, the signals from amplifiers 26 and 30 are applied to thedetector 28 roughly in quadrature, or roughly out of phase. This isaccomplished by designing the components of the amplifiers 26 and 30 inaccordance with known practice so that one exhibits roughly a 90 degreephase shift relative to the other.

Detector switching devices in the form of two diode rectifiers 110 and112 are coupled'between the ends of secondary winding 104 and a pair ofresistors 114 and 116. A capacitor 118 cooperates with resistors 1 l4and 116 to filter out any low frequency audio oscillations remaining atthe detector output, while capacitors 120 and 122 providedifferentiating and smoothing of the output signal.

In operation of the detector 28, alternating signals applied to thetransformer primary winding 102 by reference oscillator 30 tend torender diodes 110 and 112 simultaneously periodically conductive inperiods when the oscillating reference signal is positive. Thealternating composite signal applied to centertap 106 of the secondarywinding 104 tends to render the diodes 110 and 120 alternatelyconductive as the alternating signal varies between positive andnegative peaks, with resistor 124 providing a current path for flowthrough the diodes.

In steady state operation with no movement in the protected area, aconstant DC signal appears at the output of the detector 28. Whenmovement occurs, the phase of the reflected signal changes and causes achange in the phase of the composite signal coupled from the amplifier26. If the movement is toward or away from speaker 28, the Dopplereffect causes a frequency change detected by detector 28 simply as aphase shift. However, the device is not dependent on the Doppler effect.If movement equidistant to the speaker 22 occurs, no frequency shiftoccurs, but a phase shift results as the total path of travel of thesignal reflected to speaker 22 is altered. This type of phase shift isalso detected by detector 28.

A shift in phase of the composite signal relative to the referencesignal alters the periods of conductivity of the diodes 110 and 112relative to the wave forms of the applied signals and thus produces ashift or fluctuation in the detector output signal. As described below,if such fluctuations are of a frequency characteristic of the movementof an intruder desired to be detected and if they are of sufficientmagnitude and duration, then an alarm indication is produced by theintruder alarm 20.

Although similar in some respects to high frequency devices such as, forexample, FM discriminators, radar phase detectors and the like, thedetector 28 has been designed in novel fashion to operate at a low audiofrequency to produce an output signal useful in connection with thealarm 20. The composite signal is coupled through the capacitor 108rather than through a transformer winding or the like, and capacitor 108is balanced, or unpolarized, in order to prevent distortion of thesignal. In addition, resistor 124 is used in place of the inductanceused commonly at higher frequencies. Since a quantitatively accurate,balanced signal is not required, the single capacitor 118 is usedinstead of two discrete capacitors, and ground is applied directly tothe cathode of the diode 112.

Output signals from the phase sensitive detector 28 are coupled throughthe impedance matching amplifier 32 to the low band pass device 34.Amplifier 32 includes a transistor 130 connected in emitter-followerconfiguration serving the functions of providing isolation between thelow band pass device 34 and the phase sensitive detector 28 and ofestablishing the signal for driving the device 34 at the desired level.Transistor 130 is biased to the desired operating condition by biasresistors 132, 134, 136 and 138 coupled as illustrated between thetransistor electrodes, a source of positive reference potential, andground.

In order to limit operation of the intrusion alarm 20 to situationswherein the motion detected is characteristic of the type made by anintruder desired to be protected against, the low band pass device 34 isdesigned to pass a narrow band of frequencies. In the illustratedarrangement, the device 34 passes a band of frequencies having a low endat the approximate level of a few tenths of a hertz and an upper end atthe level of approximately or 6 hertz. As a result, for example, ifextremely rapid movements are detected within the protected area, suchas might be made by a fan or vibrations or the like but which are notcharacteristic of a human intruder, the resulting signals are notcoupled through the low band pass device 34.

At this point it should be noted that an additional important feature ofthe use of a relatively low audio frequency for the intrusion alarm 20is that it permits operation with a very narrow band pass such as theapproximately five hertz band pass achieved with the device 34. Withultrasonic, microwave or radio frequency devices, the minimum band passobtainable is in the area of several hundred hertz. As a result, thesignal-tonoise ratio obtained with the present invention far exceedsresults which have been obtainable in the past.

Referring more specifically now to the construction of the device 34, inthe illustrated arrangement the low band pass device 34 is an activefilter including an integrated circuit operational amplifier 140.However, it should be understood that it would be possible to use apassive filter device preferably having sharp drop-off characteristics,although it is believed that better re-' sults can be obtained with theillustrated active filter circuit. In the illustrated arrangement,operational amplifier 140 comprises a General Electric model PA230integrated circuit. If desired, any similar device such as a type LM741integrated circuit manufactured by Fairchild Camera and Instrument Corp.and by Texas Instruments Incorporated may be substituted.

Signals coupled from the impedance matching amplifier 32 are received atthe input terminals 12 and of amplifier 140, and output signals appearat output terminal 7. Stabilizing capacitors 142, 144 and 146 areprovided in the conventional manner while the amplifier is protectedagainst excessive input signals by diodes 148 and 150. The amplifier 140is energized by coupling of terminal 3 to a source of operating voltage.

In order to limit the pass band of amplifier 140, there is provided afeedback circuit including one branch composed of a resistor 152 and aparallel branch including a resistor 154 and a capacitor 156 coupledbetween output terminal 7 and input terminal 10. Input terminal 10 isalso coupled to ground by a circuit including a capacitor 158 and a gaincontrol variable resistor 160. Additional output filtering is providedby a resistor 162 and capacitor 164 coupled between output terminal 7and ground. The values of the components of the feedback and gaincontrol circuits are chosen in order to limit the pass band of amplifierto the desired level. As will be readily understood by those skilled inthe art, high frequency cut-off is essentially established by the valueof capacitor 156, while low frequency cut-off is substantiallyestablished by the value of capacitor 158.

Output signals from output terminal 7 of the amplifier 140 are receivedat an input terminal 166 of the gate circuit 36. In general, the gatecircuit 36 includes a gate transistor 168 operated by a negative gatesignal provided by either of two branches of a novel signal processingcircuit generally designated as 170 coupled between the gate circuitinput terminal 166 and the gate transistor 168.

More specifically, the gate transistor 168 is normally maintained in anonconductive condition by means of a positive voltage applied to itsbase electrode through a bias resistor 172. When output signals arereceived from the low band pass device 34, the gate transistor 168 isplaced in a conductive condition.

The novel signal processing circuit 170 functions to supply a negativegate signal necessary to render the gate transistor 168 conductiveregardless of whether the signal appearing at the gate input terminal166 is positive or negative. Thus, when a negative signal is received atterminal 166, the signal is coupled through a capacitor 174, resistor176 and diode 178 to the base electrode of gate transistor 168.Positive-going signals are blocked by diode 178 and are coupled to thesource of positive potential through a diode 180 and a load resistor182. The capacitor 174 and the diodes 178 and 180 establish a so-calledvoltage doubling circuit, and capacitor 174 is charged when the inputsignal is positive and this charge adds to negative signals when theinput signal is negative.

When a positive signal is received at the input terminal 166, it iscoupled through a capacitor 184, resistor 186 and diode 188 to the baseelectrode of an inverter transistor 190. Transistor 190 is normallymaintained in a nonconductive condition, but when biased into conductionby a positive signal at the input terminal 166 the transistor 190establishes a low resistance connection to ground through itscollector-emitter circuit and a resistor 192 in order to placethe gatetransistor 1 68 in a conductive condition. During the receipt of anegative signal at the input terminal 166, the signal is coupled toground through a diode 194 and load resistor 196, and is blocked bydiode 188 from the base electrode of the transistor 190. Diodes 188 and194 and capacitor 184 establish a voltage doubler circuit so that duringreceipt of negative signals a charge is imposed upon capacitor 184,which charge adds to a subsequent positive-going signal. The values ofthe components of the branch including transistor 190 are preferablychosen to compensate for the gain of transistor 190 to the end that thegate transistor 168 is operated in identical fashion by either negativeor positive signals. Thus, the signal processing circuit 170 functionsin a manner analogous to that of a full-wave rectifier but eliminatesthe need for a coupling transformer or other large and expensivecomponents.

During those periods of time in which the gate transistor 168 isrendered conductive, current flows through a resistor 198 to theintegrator 38 comprised of a capacitor 200 and resistor 202. Signalsoperating the gate transistor 168 as a result of detected motioncharacteristic of the type of intruder sought to be protected againstare time-integrated by the integrator 38. The value of the capacitor 200in relation to the value of the resistor 198 is chosen so that after apredetermined time of operation of the gate transistor 168, apredetermined charge level is established on the capacitor 200 in orderto initiate an alarm condition. The resistor 202 is selected to providea discharge path such that the level of charge on the capacitor 200 isdissipated in inactive periods to make the device immune to trivialwidely spaced disturbances.

When the integrator 38 is charged to a predetermined level indicating analarm condition, the relay control circuit 40 is operated in order tooperate the alarm circuit 42. More specifically, a positive voltage ofpredetermined magnitude appearing across the capacitor 200 operates anemitter-follower transistor 204 having its emitter coupled to groundthrough a resistor 206 and coupled to the base electrode of an invertertransistor 208 through a resistor 210. The emitterfollower transistor204 isolates the integrator 38 from operation of the relay controlcircuit 40.

To provide fail-safe operation, the alarm circuit includes a normallyopen set of relay contacts 212 held in closed condition during operationof the intrusion alarm when no alarm condition is experienced by steadystate encrgization of a relay winding 214. More specifically, a relaycontrol transistor 216 is normally maintained in a conductive conditionby a positive operating voltage applied to its collector electrodethrough a resistor 218 and the winding 214 and by a positive controlvoltage applied to its base electrode through a resistor 220 and aresistor 222. When an alarm condition takes place, the invertertransistor 208 is rendered conductive by operation of theemitterfollower transistor 204 with the result that the base electrodeof the relay control transistor 216 is coupled to ground by the invertertransistor 208. As a result, the relay control transistor 216 is biasedto a nonconductive condition and current flow through the relay winding214 is discontinued. At thistime, relay contacts 212 of the alarmcircuit 42 move to their normally open condition and as a result analarm condition is established in the alarm circuit 42. It will beunderstood that the alarm circuit 42 may comprise any conventional alarmequipment such as a telephone-coupled remote indicator, a visible alarm,an audible alarm, or any other desired structure.

In order to provide a visual indication of the operation of the relaycontrol circuit 40 for initial setup of the intrusion alarm 20 or thelike, there is provided an indicator light 224 controlled by a lightamplifier transistor 226. Normally the transistor 226 is maintained in anonconductive condition by connection of its base electrode through aresistor 228 to the collector electrode of the normally conductivetransistor 216. When an alarm condition takes place, the transistor 226is biased to a conductive condition wherein the transistor 216 becomesnonconductive, and current for energization of the indicator light 224flows through a resistor 230 and the output electrodes of the transistor226.

In the illustrated intrusion alarm 20, the component values and otherspecific descriptions set forth below were found to provide excellentresults. However, it should be understood that this information isprovided for completeness of disclosure and should not be taken to limitthe scope of the invention;

Component Description Resistor 46 68 ohms 48 6,200 ohms 50 6,200 ohms 526,200 ohms 62 3,300 ohms 64 1 10,000 ohms 66 30,000 ohms 68 220,000 ohms72 120,000 ohms 78 100,000 ohms 80 22,000 ohms 82 62 ohms 88 100,000ohms 90 15,000 ohms 92 82 ohms 114 1,000,000 ohms 116 1,000,000 ohms 12415,000 ohms 132 1,500 ohms 134 15,000 ohms 136 680,000 ohms 138 820,000ohms 152 1,000,000 ohms 154 10,000 ohms 162 47,000 ohms 172 120,000 ohms176 15,000 ohms 182 120,000 ohms 186 18,000 ohms 192 18,000 ohms 19647,000 ohms 198 47,000 ohms 202 270,000 ohms 206 18,000 ohms 210 18,000ohms 218 620 ohms 220 18,000 ohms 222 27,000 ohms 228 220,000 ohms 230510 ohms Capacitor 54 0.047 microfarad 56 0.047 microl'arad 58 0.047microfarad 60 0.047 microfarad 70 0.001 microfarad 74 0.047 microfarad84 270 microfarads 94 0.33 microfarad 108 10 microfarads (unpolarized)118 1 microfarad 25 microfarads 122 0.47 microfarad 142 82 'picofarads144 22 microfarads 146 22 microfarads 156 0.047 microtarad 158 270microfarads 164 15 microfarads 174 40 microfarads 184 40 microfarads 20015 microfarads Variable resistor 160 220 ohms 2,720 ohms Transistor Type44 2N5306 76 HS5308 86 HS5308 2N2924 168 2N3638A 190 2N2924 204 2N2924208 2N5306 216 2N5306 226 2N5306 Diode 110 1N4001 112 1N4001 148 1N400l1N4001 178 1N4001 180 1N4001 188 1N4001 194 1N4001 The power supply forthe intrusion alarm 20 may be conventional in structure. In manyinstallations it may be desirable to operate the alarm on battery powerwhen conventional sources are interrupted. In order to prevent falsealarms due to switching transients or other alterations in the supplyvoltage, it is preferred that a regulated power supply be used for thealarm 20 with the exception of the relay control circuit. In theillustrated device, the source of positive potential was volts suppliedfrom a regulated power supply, and the amplifier 140 was supplied with12 volts DC from a regulated power supply.

Although the present invention hass been described with reference to thedetails of the illustrated embodiment, it should be understood thatother modifications and embodiments will be apparent to those skilled inthe art. As noted above, many advantages are achieved with the use of anaudio frequency for the alarm 20. However, inaudible sonic frequencies,i.e., ultrasonic frequencies, may be a requirement in environments wheresilent operation is desirable. Features of the present invention areapplicable to systems using any sonic frequency, whether in the audiblerange or in the ultrasonic range. The details of the illustratedembodiment are not intended to limit the scope of the present inventionas set forth in the following claims.

What is claimed and desired to be protected by Letters Patent of theUnited States is:

1. An intrusion alarm for detecting the movements of an intruder in aprotectd area comprising:

signal generating means for generating an electrical signal having afrequency falling within the low audio frequency range; transducer meansfor converting electrical energy into sound energy waves in the air andfor simultaneously converting airborne sound energy waves intoelectrical energy;

phase detection means for developing an output signal whose magnitudefluctuates in response to changes in the phase relationship of twoincoming signals;

first circuit means for conveying the signal generated by said signalgenerating means to said transducer means; second circuit means forconveying a signal from said transducer means to said phase detectionmeans;

third circuit means for conveying a signal generated by said signalgenerating means directly to said phase detection means;

amplification and filtering means connected to said phase detectionmeans for amplifying only those sub-audio frequency output signalssupplied by said phase detection means having frequencies falling withina frequency band of less than four octaves the center frequency of whichband is adjusted to encompass the primary output signal frequenciesgenerated by said phase detection means when a human moves adjacent saidtransducer means;

rectification and integration means connected to the output of saidamplification and filtering means for integrating the strength andduration of signal perturbations which appear at the output of saidamplification and filtering means; and

means coupled to said rectification and integration means for producingan alarm indication in response to the achievement of a predeterminedintegral value by said integration means.

2. An intrusion alarm in accordance with claim 1 wherein said transducermeans comprises at least one loudspeaker having a single pair ofelectrical terminals.

3. An intrusion alarm in accordance with claim 1 wherein the electricalsignal generated by the generating means has a frequency between and1,000 Hertz.

4. An intrusion alarm in accordance with claim 1 wherein the electricalsignal generated by the generating means has a frequency between 400 and600 Hertz.

5. An intrusion alarm in accordance with claim 1 wherein said first andsecond circuit means include limiter means for limiting the amplitudefluctuations in signals conveyed between said signal generating meansand said phase detection means over said first and second circuit means,whereby said phase detector means is rendered relatively insensitive tosignal amplitude fluctuations.

6. An intrusion alarm in accordance with claim 1 wherein the first andsecond circuit means, the transducer means, and the third circuit meanshave phase shift characteristics chosen to cause signals flowing fromsaid signal generating means over said first and second circuit means tosaid phase detection means to be approximately in quadrature withsignals generated by said signal generating means and flowing over saidthird circuit means to said phase detection means.

7. An intrusion alarm in accordance with claim 1 wherein the firstcircuit means includes saturated signal amplification means which limitthe amplitude of electrical signals supplied to said transducer means.

8. An intrusion alarm in accordance with claim 1 wherein saidrectification and integration means includes at its input full waverectification means for supplying signal perturbations of eitherpolarity at the output of said amplification and filtering means to saidintegration means as perturbations of the same polarity so that eitherpositive or negative perturbations may be summed by said integrationmeans.

9. An intrusion alarm in accordance with claim 1 wherein saidamplification and filtering means comprises an active filter having afrequency bandwidth of approximately 3 octaves.

10. An intrusion alarm comprising in combination:

an oscillator operating at a frequency between about 400 and about 600Hertz; i

a speaker;

a first amplifier coupling said oscillator to said speaker;

a phase sensitive detector including a transformer having a primarywinding and a tapped secondary winding;

a second amplifier coupling said oscillator to said primary winding;

means including'said first amplifier coupling both said speaker'and saidoscillator to the secondary winding tap;

a sub-audio frequency band pass filter having a bandwidth in theneighborhood of three octaves and a center frequency of between 1 and 5Hertz coupled to the output of said phase sensitive detector; and

alarm means coupled to the output of said band pass filter.

11. The alarm of claim 10 further comprising fullwave rectification andintegrating means coupled between said band pass filter and said alarmmeans.

12. An intrusion alarm for detecting the movements of an intruder in aprotected area comprising:

means including a tone generator and a transducer for generating a lowaudio frequency standing wave in the protected area;

means for generating an electrical signal whose amplitude changesrepresent phase changes in said standing wave;

means for filtering said electrical signal to eliminate frequencycomponents lying outside of a predetermined sub-audio pass band;

electrical signal integration means for integrating incoming electricalsignals;

a first circuit path including a unidirectional conductive deviceoriented in a first direction over which the filtered electrical signalis conveyed to said integration means;

a second circuit path including a unidirectional conductive deviceoriented in a second direction and also including inversion means forinverting the polarity of signals flowing over said second path overwhich the filtered electrical signal is conveyed to said integrationmeans;

and alarm means coupled to said integration means for sounding an alarmwhen said integration means reaches a predetermined integral state inresponse to signals flowing over said circuit paths.

13. An intrusion alarm in accordance with claim 12 wherein each of saidfirst and second circuit paths includes voltage doubling rectificationmeans, of which the unidirectional conductive device forms a part, forrectifying signals so as to produce increased rectified signal outputfrom each path.

1. An intrusion alarm for detecting the movements of an intruder in aprotectd area comprising: signal generating means for generating anelectrical signal having a frequency falling within the low audiofrequency range; transducer means for converting electrical energy intosound energy waves in the air and for simultaneously converting airbornesound energy waves into electrical energy; phase detection means fordeveloping an output signal whose magnitude fluctuates in response tochanges in the phase relationship of two incoming signals; first circuitmeans for conveying the signal generated by said signal generating meansto said transducer means; second circuit means for conveying a signalfrom said transducer means to said phase detection means; third circuitmeans for conveying a signal generated by said signal generating meansdirectly to said phase detection means; amplification and filteringmeans connected to said phase detection means for amplifying only thosesub-audio frequency output signals supplied by said phase detectionmeans having frequencies falling within a frequency band of less thanfour octaves the center frequency of which band is adjusted to encompassthe primary output signal frequencies generated by said phase detectionmeans when a human moves adjacent said transducer means; rectificationand integration means connected to the output of said amplification andfiltering means for integrating the strength and duration of signalperturbations which appear at the output of said amplification andfiltering means; and means coupled to said rectification and integrationmeans for producing an alarm indication in response to the achievementof a predetermined integral value by said integration means.
 2. Anintrusion alarm in accordance with claim 1 wherein said transducer meanscomprises at least one loudspeaker having a single pair of electricalterminals.
 3. An intrusion alarm in accordance with claim 1 wherein theelectrical signal generated by the generating means has a frequencybetween 100 and 1,000 Hertz.
 4. An intrusion alarm in accordance withclaim 1 wherein the electrical signal generated by the generating meanshas a frequency between 400 and 600 Hertz.
 5. An intrusion alarm inaccordance with claim 1 wherein said first and second circuit meansinclude limiter means for limiting the amplitude fluctuations in signalsconveyed between said signal generating means and said phase detectionmeans over said first and second circuit means, whereby said phasedetector means is rendered relatively insensitive to signal amplitudefluctuations.
 6. An intrusion alarm in accordance with claim 1 whereinthe first and second circuit means, the transducer means, and the thirdcircuit meAns have phase shift characteristics chosen to cause signalsflowing from said signal generating means over said first and secondcircuit means to said phase detection means to be approximately inquadrature with signals generated by said signal generating means andflowing over said third circuit means to said phase detection means. 7.An intrusion alarm in accordance with claim 1 wherein the first circuitmeans includes saturated signal amplification means which limit theamplitude of electrical signals supplied to said transducer means.
 8. Anintrusion alarm in accordance with claim 1 wherein said rectificationand integration means includes at its input full wave rectificationmeans for supplying signal perturbations of either polarity at theoutput of said amplification and filtering means to said integrationmeans as perturbations of the same polarity so that either positive ornegative perturbations may be summed by said integration means.
 9. Anintrusion alarm in accordance with claim 1 wherein said amplificationand filtering means comprises an active filter having a frequencybandwidth of approximately 3 octaves.
 10. An intrusion alarm comprisingin combination: an oscillator operating at a frequency between about 400and about 600 Hertz; a speaker; a first amplifier coupling saidoscillator to said speaker; a phase sensitive detector including atransformer having a primary winding and a tapped secondary winding; asecond amplifier coupling said oscillator to said primary winding; meansincluding said first amplifier coupling both said speaker and saidoscillator to the secondary winding tap; a sub-audio frequency band passfilter having a bandwidth in the neighborhood of three octaves and acenter frequency of between 1 and 5 Hertz coupled to the output of saidphase sensitive detector; and alarm means coupled to the output of saidband pass filter.
 11. The alarm of claim 10 further comprising full-waverectification and integrating means coupled between said band passfilter and said alarm means.
 12. An intrusion alarm for detecting themovements of an intruder in a protected area comprising: means includinga tone generator and a transducer for generating a low audio frequencystanding wave in the protected area; means for generating an electricalsignal whose amplitude changes represent phase changes in said standingwave; means for filtering said electrical signal to eliminate frequencycomponents lying outside of a predetermined sub-audio pass band;electrical signal integration means for integrating incoming electricalsignals; a first circuit path including a unidirectional conductivedevice oriented in a first direction over which the filtered electricalsignal is conveyed to said integration means; a second circuit pathincluding a unidirectional conductive device oriented in a seconddirection and also including inversion means for inverting the polarityof signals flowing over said second path over which the filteredelectrical signal is conveyed to said integration means; and alarm meanscoupled to said integration means for sounding an alarm when saidintegration means reaches a predetermined integral state in response tosignals flowing over said circuit paths.
 13. An intrusion alarm inaccordance with claim 12 wherein each of said first and second circuitpaths includes voltage doubling rectification means, of which theunidirectional conductive device forms a part, for rectifying signals soas to produce increased rectified signal output from each path.